1. Field of the Invention
The present invention is in the field of manufacturing boron and phosphorus doped silicon dioxide layers which are employed in the manufacture of integrated semiconductor circuits, as intermediate layers for electrical insulation and for the compensation of topographical irregularities on substrate wafers.
2. Description of the Prior Art
Boron and phosphorus doped silicon oxide layers are referred to as boron-phosphorus-silicate glass (BPSG) layers and are manufactured by oxidizing silane in the temperature range between 300.degree. and 450.degree. C. To generate the oxide doping, phosphine (PH.sub.3) and diborane ) (B.sub.2 H.sub.6) or boron trichloride (BCl.sub.3) are simultaneously added. The process can be carried out both in a plasma arc at atmospheric pressure or at a reduced pressure.
Deposition methods for use at atmospheric pressure and in the aforementioned temperature range using phosphine and borane as doping gases are described, for example, in U.S. Pat. No. 3,481,781 to Kern and in an article by Kern et al in the RCA Review, Vol. 43, September 1982, pages 423 to 457.
In their application as intermediate oxide layers between polysilicon levels and an aluminum interconnect level, the boron-phosphorus-silicate glass layers must meet the following conditions for the manufacture of VLSI MOS memories:
1. The intermediate oxide must guarantee good electrical insulation between the polysilicon level or the diffusion regions and the aluminum interconnect level.
2. The intermediate oxide level must level off or round off the topographical irregularities on the substrate which arise during the polysilicon structuring. Such a rounding off is necessary since the subsequent aluminum sputtering process exhibits shadowing effects and could effect excessively pronounced steps in the substrate, overhanging portions, or contouring of the aluminum interconnects. This problem becomes more critical with increasing integration of components since as the lateral dimensions decrease with unchanged layer thicknesses, the height/breadth aspect ratio increases.
Since only an edge covering which does not exhibit additional overhangs can be achieved by means of optimizing the oxide deposition process utilizing chemical vapor deposition (CVD), it is necessary to use an intermediate oxide process which is a combination of layer deposition and subsequent "flowing". In order to carry out such a flowing step at temperatures which are realistic in terms of process engineering, the SiO.sub.2 must contain additives which reduce its softening point. In practice, phosphorus or boron and phosphorus are employed for this purpose, these materials being incorporated into the intermediate oxide layer during the layer deposition in the form of their oxides and forming what are known as "ternary" phases. The phosphorus has the primary function of improving the electrical stability of the layer and the boron content controls the flow temperature.
The processes mentioned above from the prior art for the manufacture of boron-phosphorus-silicate glass layers have various disadvantages.
1. As a source of boron, diborane is toxic, explosive and chemically unstable. The diborane content of a gas bottle containing the diborane gas mixture decreases in time and so no traditional low pressure gaseous phase deposition utilizing tube systems with injectors can be employed since the diborane heats and decomposes in the injectors. Boron trichloride, on the other hand, may lead to an incorporation of chlorine in the layer, causing serious corrosion problems of the subsequent metallization layers which are applied.
2. The chemical stability in the presence of atmospheric humidity is generally poor with the doped low temperature oxides. For example, boric acid crystals may occur when the boron content is in the range from 3 to 4 weight percent, as disclosed in the initially cited report by Kern and Schnable from the "RCA Review", Vol. 43, 1982, pages 423 through 457.
3. The phosphorus in the low temperature layers is partly present as P.sub.2 O.sub.3 and partly as P.sub.2 O.sub.5, whereas only the pentavalent phosphorus is responsible for the flowing properties.
4. Oxide processes based on silane are very sensitive to leaks in the deposition system.
5. The edge coverage of the low temperature oxides is generally unsatisfactory.